PAH destruction at ionization fronts

Details PAH destruction at ionization fronts PAH destruction at ionization fronts

Nov 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994a%26a...291..239g&link_type=abstract

Astronomy and Astrophysics (ISSN 0004-6361), vol. 291, no. 1, p. 239-249

Astronomy and Astrophysics

Astronomy

53

Cosmic Dust, Infrared Astronomy, Interstellar Gas, Ionized Gases, Molecular Clouds, Nebulae, Polycyclic Aromatic Hydrocarbons, Angular Resolution, Emissivity, Infrared Radiation, Line Of Sight, Opacity

Scientific paper

We have mapped the Orion bar and the M17 South-West interface in the 3.3 micrometer dust feature and the adjacent continuum with a 1 sec angular resolution and an unprecedented sensitivity. The 3.3 micrometer images reveal the clumpy structure of the gas in the photodissociation region down to scales of a few 10-3 pc. The 3.3 micrometer emitting particles are strictly confined in the neutron/molecular gas just behind the ionization/shock fronts, with no detectable emission from the ionized gas. The transition at the ionization front is not resolved, showing that the 3.3 micrometer emitting particles are destroyed in the HII gas at a rate larger than2.7 10-11/s. More than the intensity of the radiation field, the ionized gas density is the physical parameter which shows the largest variation at the ionization front. This is why, in the frame of the Polycyclic Aromatic Hydrocarbon (PAH) model, we propose that H+ chemisputtering in the HII gas is responsible for the destruction of the molecules. However, other destruction mechanisms such as the photo-thermodissociation of PAH can contribute, and, in conjunction with an efficient formation process operating mostly in the dense neutral/molecular gas, could explain the observations. Concerning the continuum measurements, we show it cannot be explained by the free-free emission of the ionized gas. Two components are observed. The first one is spatially correlated with the 3.3 micrometer feature, and may be due to the same kind of particles. The second one is correlated with the HII gas, exhibits a 3.9 to 2.2 micrometer color ratio Flambda(3.9)/Flambda(2.2) approximately = 0.81, and is attributed to warm dust grains. The absence of color variations correlated with the radiation field intensity argues for transiently heated very small grains in the HII regions. Finally, seven extremely red sources with K-L colors larger than 3.5 are detected in the M17 south-west molecular cloud.

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